The invention relates generally to the field of data communication. More particularly, this invention relates to a technique for establishing a channel band plan for a node in a communication network that utilizes multi-channel hopping. This invention has particular application to packet communication radio mesh networks using frequency hopping.
Packet communication networks provide for the transfer of data packets between various remote locations herein referred to as nodes. Nodes are equipped with transmitters and receivers that can transmit data over a medium, which may be radio waves, fiber optic cable, wire, etc. In order to accomplish successful data transfer, each node must operate in accordance with a protocol determining when it may transfer on the medium, for how long, and, in a multiple channel network, on what channel. Additional protocol tasks include error checking and error correction. Various network protocols and network topologies are discussed in earlier related patents in this field owned by the assignees of the present invention, including U.S. Pat. Nos. 5,115,433; 5,007,052; and other patents mentioned below.
U.S. Pat. No. 5,079,768 (Flammer et. al) issued to one of the inventors of the present invention, describes a peer-to-peer frequency-hopping radio-based communication network in which every receiver node is assigned a different starting point or home slot in a frequency hopping pattern. The frequency-hopping pattern is a randomized ordered list of all the channels available to nodes in the network. The channel order is shared by all the nodes in the network, but contention is reduced by having each node start its channel hopping at a different channel in the list. Transmitters wishing to transmit to a receiver node must switch to that receiver's current frequency in the hopping pattern to transmit a poll packet. The transmitter first listens at the assigned frequency of the receiver to determine if there is any traffic at that frequency. If there is traffic at that frequency, the transmitter waits an interval and the transmitter and receiver both hop to the next frequency in the pattern and the transmitter listens again. Once a frequency is found with no traffic, the transmitter sends a poll packet, and when that poll packet is acknowledged, the transmitter sends a data packet before the hop to the next frequency. In a particular embodiment of this network, frequency hopping occurs once every second.
As discussed the '768 patent, communication on certain frequencies may be restricted in duration in accordance with frequency allocation and bandwidth utilization rules and requirements. Such restrictions may be imposed by a licensing authority, such as the U.S. Federal Communications Commission (FCC). For example, in the 902-928 MHz frequency band, the FCC has proposed a rule that continuous transmission by a single transmitter of no more than 1 watt rf output power on any one channel be of no more than 400 ms duration each 30 seconds, and that at least some if not all other channels be selected prior to retransmission on the same frequency (FCC Rules, Part 15.247). Communication between any given pair of transceivers on a single frequency is thus restricted to packets of information which can be communicated in less than 400 ms, and means must be provided to accommodate communication on other frequencies.
Related co-pending U.S. patent application Ser. No. 08/193,338, claims improvements to the frequency-hopping communication network of Flammer. According to the invention disclosed in that application, when a target node returns an acknowledgement, the target node reserves access to itself for the polling station at a preselected time for a preselected duration on a specified frequency channel that is different from its assigned channel according the band plan. The source node then transmits its data packet on the target node's data receive channel and waits for an acknowledgement on the same receive channel. The source node and the target node exchange information on the same channel throughout the interchange, even though the assigned receive channel of the target node according to the frequency-hopping band plan may have changed in the meantime. With this improvement, collisions between the data packet and other acquisition packets directed to that target node are avoided because all other nodes will direct poll packets to the target at the frequency according to the target's band plan, while the target remains at the data exchange frequency.
Performance of the network described in the '768 patent is limited in two respects. One is due to collisions that may arise when two nodes happen to start out at the same home slot. With the identical frequency-hopping pattern shared by all nodes in the network, two nodes that start out at the same home slot frequency will always hop together and remain on the same frequency. In a specific embodiment of the network described in the '768 patent, there were just 205 channels available. In a mesh network with a large number of nodes, having two nodes share the same starting frequency is a not unlikely occurrence. According to the network protocol, when a collision occurred, the transmitting nodes wait and retransmit at a subsequent frequency hop. If two nodes where hopping on the same band plan, and one node was very busy, an appreciable number of data packets would be lost and would need to be repeatedly transmitted due to repeated collisions.
A second factor limiting performance in the Flammer network is that the fixed frequency hopping sequence did not allow transmitters to skip channels on which the receivers could not receive data. This problem is particularly acute when nodes are located in possibly disparate locations throughout a geographic area. Since these nodes operate in the radio frequency spectrum, they are dependent upon it for the propagation of the data carrying signals between the nodes. At each particular location, the frequency spectra can have a unique level and pattern of spectral occupancy. Optimum network performance would dictate that those channels that cannot support robust communication at the time not be "bothered with"; unusable frequencies should be seemlessly eliminated, newly useable frequencies should be seemlessly integrated into the frequencies carrying data traffic. However, in the network disclosed in the '768 patent, all nodes were required to hop through all the available channels in the network, even if a particular node could not receive data on that channel.
What is needed is a network with a low-cost, reliable mechanism for establishing a unique channel hopping band plan for each node and that allows individualized seamless elimination of inoperable channels from a particular node's band plan.